The thermal properties of the soil (calorific capacity, specific heat, thermal conductivity, and thermal diffusiveness) vary with water content, granulometry, and aggregation of the soil (Porta, C. J., López-Acevedo, R. M. y Roquero, C. Edafología para la agricultura y el medio ambiente. Ed. Mundi-Prensa, Madrid, España, p. 807, 1994) and are related with microbial activity, mineralization, and humification of the organic matter (Montenegro y Malagón. Propiedades fisicas de los suelos, 2 ed. IGAC, Bogotá D.C. Colombia, p. 813, 1990).
The thermal conductivity (λ) or ability of the soil to transfer heat through molecular conduction is expressed in calories flowing through a soil plate with unitary thickness and area, with a difference of 1° C. between the two faces (Honorato, R. Manual de Edafología. Alfaomega 4a edición. Universidad Católica de Chile, p. 267, 2000). When the thermal conductivity of a soil is high, the temperature variations on the surface are lower. The thermal conductivity is increased with the humidity content, affecting also the changes of soil temperature; however, when there is saturation of the pores, it does not increase in the expected proportions. Thermal conductivity is considered a characteristic sensitive to changes operating on the soil (Honorato, R. Manual de Edafología. Alfaomega 4a edición. Universidad Católica de Chile, p. 267, 2000; Jury, W. A. and Roth, K. Transfer functions and solute movement through soil. Theory and applications. 1 ed. Basel: Birkhäuser Verlag Basel, p. 226, 1990).
Numerous devices to determine thermal conductivity are known in literature, for example U.S. Pat. No. 4,861,167 presents an apparatus to determine thermal conductivity of fused polymers from the rate of temperature change in the sample. The apparatus is made up of an elongated cylindrical container that holds the sample, heating means located around the container, a temperature sensor, a probe that can inserted into the container which contacts the sample and which has a hollow needle, a heating medium and a sensor located on the body of the needle and means connected to the sensor to determine the change in temperature in the sample that is in contact with the probe. Also, U.S. Pat. No. 3,263,485 shows an apparatus to determine the thermal conductivity of a solid material through a method that compares the unknown value of conductivity of a sample against a material of known low thermal conductivity; the apparatus has a covered receptacle on the internal walls by isolating material, a cylindrical tube mounted inside the receptacle, a source of heat placed on one of the extremes of the tube, a plurality of heating wires rolled helicoidally on a corrugated surface on the external wall of the tube, and sensors to measure the temperature in fixed points of each of the samples.
In spite of the existence of the previously described, there is still the need for a device to directly determine the thermal conductivity of a solid sample like the soil (and not comparatively) during a relatively short processing time and based on a reliable methodology stemming from exactness and precision of the method.
The device to determine thermal conductivity of the invention is based on the thermo-electric method, which permits studying diverse phenomena with a high range of reliability through the thermal behavior of materials, solving the drawbacks of the previous art by means of a device constituted by a sample holder cylinder surrounded by a resistance that creates a radial flow of heat on the sample and a refrigeration system based on a heat exchanger in spiral shape integrated to the thermal device. This refrigeration system based on a heat exchanger in spiral form integrated to the device diminishes the processing time for each sample, increasing the number of determinations of thermal conductivity per unit of time; for which when increasing the efficiency in the process, there is also an increase in the number samples analyzed by the object of study offering greater information of such. Additionally, this device can be used for the analysis of the organic quality of foods and agricultural products.